Dental materials generally have unique requirements as compared to the broad spectrum of composite materials. For health reasons, dental materials should be suitable for use in the oral environment. In certain instances, strength and durability of a dental material is important to ensure satisfactory performance. For example, in dental work that is performed at dentition locations where mastication forces are generally great, high strength and durability is desirable. In other instances, aesthetic character or quality (e.g., luster and translucency) is highly desired. This is often the case where dental work is performed at locations where a tooth repair or restoration can be seen from a relatively short distance.
Strength in a dental material is typically achieved by adding fillers. Generally, a dental material possessing greater mechanical strength characteristics is filled or loaded with larger sized particles; i.e particles having a diameter greater than about 0.4 micrometers. These materials are often referred to as hybrid composites. A disadvantage to these composites, however, is their tendency to lack luster and aesthetic character. Another disadvantage of composites with large-sized particles is that with repeated toothbrushing (a requirement for oral hygiene), the hardened resin can wear away, exposing the large filler particles and leave a dull, unaesthetic surface. This can subsequently lead to plaque accumulation.
Increasing filler levels can also increase the strength of a dental material. However, this can lead to increased visual opacity, thereby reducing translucency and aesthetic quality.
Canadian Patent Application 2,202,732 teaches polymerizable dental materials comprising a sol of surface modified silica particles in a liquid, organic dispersion agent. The silica particles comprise about 35 wt % of the dental material.
Good rheological properties in unhardened dental materials are advantageous to a dental practitioner. This allows the practitioner to easily manipulate and place the material in its desired location and achieve proper contact and anatomical form before hardening or curing. Nanometer sized (“nano-sized”) silica particles, most often in the form of fumed silica, have been dispersed in polymerizable dental resins. A fumed silica material available from DeGussa, under the trade designation OX-50 (DeGussa AG, Hanau, Germany), has had widespread use. Materials made with fumed silica dispersed at high loading levels within the resins, however, result in dilatant compositions that are generally impractical for dental practice. A well-recognized dental reference book by Craig, entitled, “Restorative Dental Materials,” 8th ed., 1989 teaches that highly-loaded fumed silica materials generally provide materials with poor theological properties. (See e.g., p.256 of Craig.) Thus, conventional materials whose concentrations of an inorganic component (particles) are adjusted for a desired strength, typically result in undesirably dilatant materials.
It has also been the practice to incorporate pre-polymerized particles to overcome the dilatant rheology. These, however, can result in low strength materials.
It is generally desired that the dental material blends well with the surrounding dentition and looks life-like. Aesthetic quality in dental materials is typically achieved by creating material that has tooth-like colors/shades. “Microfills,” a certain class of dental materials, tend to have good luster to better replicate tooth appearance. One example of a “microfill” is commercially available under the trade designation SILUX PLUS (3M Co., St. Paul, Minn.). Microfills, however, generally have less mechanical strength than hybrid composites or “macrofills.”
Thus, in current practice, for applications where high strength and high aesthetic quality are desired, a practitioner is typically required to first use an underlying foundation of a material possessing high physical strength followed by an overlying layer of a microfill. It would be advantageous to provide a single material that possesses high strength and high aesthetic quality.